Improving Energy Efficiency of Wastewater Residue Biomass Utilisation by Co-Combustion with Coal

Abstract

The accelerated urbanisation that is occurring in many regions of the world is resulting in a corresponding increase in the volume of sewage sludge. This sludge is then stored in specialised landfills, the area of which is increasing annually. One of the methods of utilising this sludge is through its combustion in power plants, where it serves to generate heat. However, due to the low calorific value of sewage sludge, it is recommended to combust it in conjunction with high-calorific fuel. To improve energy efficiency of sewage residue biomass utilisation by co-combustion with coal, it is necessary to determine the main combustion parameters and mass fraction in the mixture. The objective of this study is to estimate the primary parameters of combustion of sewage sludge and coal by employing the synchronous thermal analysis method, in addition to determining the concentrations of gaseous substances formed during the combustion process. A comprehensive technical and elemental analysis of the fuels was conducted, and their thermal properties were thoroughly determined. The inorganic residue from sewage sludge combustion was analysed by scanning electron microscopy for the content of trace elements and basic oxides. Thermogravimetric analysis (TGA) of fuels was conducted in an oxidising medium, utilising a 6 mg suspension with a heating rate of 20 °C/min. The profiles of TG, DTG, and DSC curves were then utilised to determine the ignition and burnout temperatures, maximum mass loss rate, combustion index, and synergistic effects. The mixture of coal with 25% sewage sludge was found to have the most energy-efficient performance compared to other mixtures, with a 3% reduction in ignition temperature compared to coal. Concentrations of carbon dioxide, carbon monoxide, nitrogen oxides, and sulphur oxides were also determined.

1. Introduction

Coal is the world’s primary energy fuel, and its advantages over other fuels are its low cost, availability, and relatively high calorific value. In 2023, the following countries were identified as having the highest coal reserves: the United States (22%), Russia (15%), Australia and China (14%), and India (11%). Collectively, these five nations possess a staggering 75% of the world’s solid fossil fuel reserves. With regard to coal consumption, China is the leader, with 50.5% of global consumption; followed by India (11.3%), the USA (8.5%), Germany (3%), and Russia (2.7%). In 2023, 84 countries committed to eliminating coal combustion for heat and power generation, as well as suspending the construction of new coal-fired power plants [1,2,3]. However, it should be noted that these sources accounted for a mere 30 percent of the global coal-fired generation. According to the reforms proposed by the Commission on Global Energy Transition and Energy Efficiency in Power Generation, it is necessary to reduce gaseous emissions that have a negative effect on climate change [4]. The primary focus of these reforms is on the increased use of renewable energy sources [5,6]. Nevertheless, oil, gas, and coal remain the dominant energy fuels in the present day. In 2022, fossil fuels accounted for 82% of total global consumption, while only 7.5% (excluding hydropower) was accounted for by renewable energy sources [7,8].

In addition to solar, wind, and other energy sources, biomass is also a low-carbon energy source. The term ‘biomass’ as a source of energy is commonly understood to refer to waste generated by the use and processing of various carbon-containing materials of plant origin, as well as various plant crops that are specially cultivated for energy production. The term ‘waste’ is understood to include residue biomass from woodworking, agricultural, food production, household waste, and sewage residue. It is a commonly held assumption that such waste is cost-free. The utilisation of biomass as a carbon-neutral fuel is considered to be in accordance with the principles of green development as outlined in global policy [9,10,11]. Current projections estimate that by 2035, biomass may contribute approximately 10% to the overall energy balance [12].

Sewage sludge (SS) is produced as a result of the biological treatment of urban sewage in cities with centralised sewerage systems. SS is a semi-solid material consisting primarily of biomass, in addition to various non-combustible inorganic substances [13]. The primary methods of SS utilisation are combustion/gasification and landfilling. However, landfilling of SS can have detrimental environmental consequences, particularly in cases of its penetration into the soil during periods of heavy rainfall [14,15]. Conversely, SS’s low calorific value renders it an ineffective fuel when utilised alone; however, when utilised in a mixture with highly reactive fuels, its combustion can be enhanced. Research conducted on the co-combustion of SS and coal sludge has demonstrated the efficacy of controlling sulfur-containing substances emitted during the process [16]. In the context of the co-combustion of SS and corn cobs, it has been determined that the most pronounced synergistic effect is attained when 70% of the biomass is incorporated into SS. Furthermore, the co-combustion process has been demonstrated to reduce the combustion temperature of SS coke residue [17]. Furthermore, the co-combustion of SS and RDF was found to be most efficient when a mixture containing 20% SS was utilised [18].

One of the most effective methods of utilising SS is through its combustion in boilers in conjunction with coal. Research conducted in Poland has demonstrated that the co-combustion of coal and SS necessitates the selection of the optimal air ratio, i.e., the necessary excess air ratio. It has been established that an increase in the concentration of SS in the mixture with coal slime results in an increase in the concentration of nitrogen and sulphur oxides in the exhaust gases [19]. According to the authors [20], a significant reduction of nitrogen and sulphur oxides in the exhaust gases was achieved at combustion of a mixture of sewage sludge and coal slime in the ratio 1:1. Concurrently, the ignition and burnout temperature decreased. Sun, Y. et al. [21] in their work investigated the combustion of SS and coal using thermal analysis and found that the increase of SS in the mixture contributes to the deterioration of stable combustion of the mixture. When 10% is added, the heat of combustion of the mixture is reduced by only 4.2% compared to coal. Zhou, A. et al. [22] analysed the generated ash and slag from three different thermal power plants combusting coal and SS jointly and found that combustion of mixtures with low SS content has little effect on the melting characteristics of ash in boilers. The high iron content in the mineral part of SS is the main reason why the melting point of SS ash is lower than that of coal.

To improve energy efficiency of sewage residue biomass utilisation by co-combustion with coal, it is necessary to determine the main combustion parameters and mass fraction in the mixture. The purpose of this work is to determine the main parameters of combustion (temperature at which the ignition of carbonaceous residue occurs; temperature at which the combustion process is completed; combustion index and others) of a solid fuel mixture based on coal and sewage sludge, taking into account the analysis of the concentrations of gaseous substances formed. The findings of this study are to be used in the planning of SS utilisation by co-combustion with coal.

2. Materials and Methods

2.1. Fuels

Coal (C) was obtained from the Krasnoyarsk boiler house. The C used for the research in the paper is burnt at thermal power plants in the production of heat and electricity at thermal power plants and in small heating boilers.

Samples of sewage sludge (SS) were collected from the Abakan landfill, a site where municipal sewage is stored after undergoing treatment at a sewage treatment plant. Following an extended period of exposure to the elements, SS undergoes significant moisture loss. Figure 1 presents a visualisation of the fuels under study.

The mixtures were obtained by adding 25% mass fraction of SS to C, resulting in the following three mixtures: 75% C + 25% SS; 50% C + 50% SS and 25% C + 75% SS.

2.2. Thermal-Technical Parameters of the Fuels Under Study

The C and SS under investigation were found to have a relatively high humidity level (30–40% for C and 20–35% for SS). However, after a certain amount of time in laboratory conditions, the fuels reached an air-dry state. Consequently, the results presented in

Table 1. Main characteristics of the fuels under study.

Fuels	Wr	Wa	Ad	Vdaf	Cdaf	Hdaf	Ndaf	Sdaf	Odaf	Qri	Qdafs
	%									MJ/kg
C	32.6	5.3	8.9	45.1	72.8	5.1	1.0	0.3	23.1	16.3	28.7
SS	35.2	6.7	56.5	83.7	56.1	6.1	6.2	1.6	30.0	9.6	22.7

illustrate the humidity levels in both analytical and working conditions. This enables the recalculation of any value to a different state. Prior to the studies, the fuels were milled to a dispersity of 100–250 µm using Retsch AS200 (Haan, Germany) and Retsch DM200 (Haan, Germany) equipment in accordance with [23] ISO 3310-1:2016 «Test sieves—Technical requirements and testing—Part 1: Test sieves of metal wire cloth». This class of fuel coarseness is applied in coal-fired thermal power plant for use in boilers with chamber furnaces and flaring [23,24]. Technical analysis of fuels included determination of moisture content using MA–150 (Sartorius, Göttingen, Germany), obtaining values in analytical and working states (W^r and W^a), ash content using muffle furnace Snol 7.2/1300 (AB ‘Umega’, Utena, Lithuania) in dry state (A^d), volatile yield in dry ash-free state (V^daf) with the same muffle furnace and heat of combustion in dry ash-free and working states (Q^r_i and Q^daf_s) with the calorimeter C6000 (IKA, Staufen, Germany). Analyses were performed according to standard ISO procedures: [24] ISO 18134-1:2022; ISO 18122:2022 “Solid biofuels—Determination of moisture content”; [25] ISO 18123:2023 and ISO 18125:2017 “Solid biofuels—Determination of volatile matter”. Carbon, hydrogen, nitrogen, sulphur contents were determined on a Vario MACRO cube device (Elementar Analysensysteme GmbH, Langenselbold, Germany) using standard methods [26] ISO 16948:2015 “Solid biofuels—Determination of total content of carbon, hydrogen and nitrogen”; [27] ISO 16994:2016 “Solid biofuels—Determination of total content of sulfur and chlorine” and [28] ISO/TS 20048-1:2020 “Solid biofuels—Determination of off-gassing and oxygen depletion characteristics”. Oxygen content was calculated by the difference: 100% − (C^daf + H^daf + N^daf + S^daf). As illustrated in Table 1, the technical and elemental composition of fuels is presented.

The SS sample exhibited a high content of volatiles (83.7%) and an ash content of 56.5%, which is consistent with the expected characteristics of this type of substance (see Table 1 for details). According to the findings of studies [13,18,19,20,25], the ash content in the studied sewage sludge samples ranges from 25% to 61%. The low heat of combustion of SS, as a consequence of its high ash content, is significantly lower than that of coal.

2.3. Analysis of Trace Element Content in the Inorganic Residue of Sewage Sludge

The content of trace elements in the inorganic SS residue was analysed by scanning electron microscopy. The mineral part of SS was carried out using scanning electron microscope TM4000 (Hitachi, Tokyo, Japan) operating at an accelerating voltage of ≈20 keV. The microscope was equipped with an energy dispersive spectrometer and the Quantax 75 X-ray microanalysis system (Bruker, Berlin, Germany), and the studies were performed under low vacuum conditions without precoating the samples with platinum. SS particles are porous organomineral conglomerates with a size range of 100–250 μm (Figure 2). Mineral inclusions (up to 50 μm) are primarily constituted by quartz, magnesium, calcium, potassium aluminosilicates, calcium sulfates and/or phosphates, and iron oxides (Figure 2a). Larger individual particles of these minerals (with the exception of plagioclases) have also been observed.

The chemical composition of inorganic residue is of interest in the context of joint combustion of C and SS, as well as the formation of complex ash and slag wastes. The chemical composition of the residue is dominated by silicon, present as quartz and silicates, calcium, found as calcite, gypsum and apatite, and iron, present as oxides and silicates. The results obtained from this study indicate that SS additions will contribute to the strong binding of significant amounts of toxic heavy metals in mobile forms capable of migration during long-term storage in natural conditions. The high phosphorus content of the additives may lead to additional binding of mobile harmful components of coal in poorly soluble phosphates.

The chemical composition of the samples was determined by X-ray fluorescence spectral analysis using an energy dispersive spectrometer S2RANGER (Bruker, Berlin, Germany). The content of elements was automatically converted to oxide form using a special programme for convenience of balance calculations. The obtained data on the content of elements (in terms of oxides) are given in

Table 2. The base metal content (in terms of oxides) in inorganic residue SS.

Ash	SiO2	P2O5	CaO	K2O	Fe2O3	ZnO	SO3	TiO2	SrO	Cr2O3	Al2O3
	%
SS	45.1	17.7	11.6	2.1	11.0	0.2	4.9	1.8	0.1	0.2	5.4

. The inorganic residue SS is represented mainly by compounds of silicon (45.1%), phosphorus (17.7%), calcium (11.6%) and iron (11.0%).

2.4. Thermogravimetric Analysis and Determination of Basic Combustion Parameters

Thermogravimetric analysis is used not only to determine some fuel parameters such as moisture content, inorganic residue content, heat of combustion, but also to determine some parameters related to fuel combustion under non-isothermal heating conditions. Non-isothermal heating of the fuel allows a more detailed study of the temperature regime of the combustion process of volatiles and coke residue. Thermogravimetric analysis was carried out using a thermoanalyser SDT Q600 (Thermal Analysis, New Castle, DE, USA) in an air current (50 mL/min) at a fuel heating rate of 20 °C/min. An experiment without samples was conducted in advance under the same conditions to construct a baseline. Each experiment lasted about 45 min. The mass of the fuel was about 6 mg. The most important indicator was the repeatability of the experimental results, which was ensured by conducting each experiment at least twice, thereby the inaccuracy was no more than 3%. The Universal Analysis 2000 (Thermal Analysis, New Castle, DE, USA) programme supplied with the instrument was used to process the profiles of mass loss curves (Weight), mass loss rate (Deriv. Weight) and heat flow intensity (Heat Flow).

The maximum value of the mass loss rate (Δm_max) is set on the Deriv. Weight, which is defined as the maximum burning rate of volatile substances and coke residue, was characterised by the same method. The ignition temperature (T_i) and burnout temperature (T_b) were determined by the method of intersection of the curves Weight and Deriv. Weight. The methodology for determining these combustion parameters is described in [26,27,28,29]. The combustion index (S, min⁻² °C⁻³) was utilised to compare the combustibility properties of fuels [30,31,32]. It was established that high values of the combustion index are indicative of highly reactive fuels. The following formula was utilised to ascertain the combustion index [30,31,32]:

$$S={\displaystyle \frac{\Delta {\mathrm{m}}_{\max }·\Delta {\mathrm{m}}_{\mathrm{mean}}}{T_i^2·T_b}}\times {10}^6, {\min }^{-2} °{\mathrm{C}}^{-3}$$

where Δm_max is defined as the value of peak fuel mass change rate (selected by the maximum value of Δm_max) or peak combustion rate, %/min. Δm_mean is expressed as the average value of mass change rate from ignition temperature to burnout temperature, %/min. Finally, T_i and T_b are defined as the temperatures corresponding to ignition and burnout, °C.

The combustion of a mixture may be governed by the principle of additivity, in which case the combustion parameters of the mixture can be determined by employing a proportion that utilises the combustion parameters of each constituent element of the mixture. However, in the absence of additivity in the combustion of a mixture, the combustion parameters can only be determined through experimentation. In order to ascertain the underlying principle governing the combustion process of mixtures, the method [33,34,35] of comparison of Der.Weig. curve profiles gained by experimental (DerWeig_exp) and calculated (DerWeig_est) methods is employed. The calculated values (DerWeig_est) were obtained using the following formula [31,32,33]:

DerWeig_est = λ₁DerWeig_coal+ λ₂DerWeig_{sewage sludge}, %/min

where DerWeig_coal and DerWeig_{sewage sludge}—values of intensity of mass change at each moment of time for coal and sewage sludge, %/min; λ₁ and λ₂—content of components in the mixture, their sums should be equal to one.

For the analysis, the profiles of Der. Weig curves obtained experimentally and calculated for each mixture separately are compared on one graph. The coincidence of the curves indicates the additivity of the mixture combustion. Synergistic effects between mixture components may cause the curves to diverge, which usually affects the maximum value of Der.Weig.

2.5. Determination of Concentrations of Main Components in Flue Gases

The process of combustion, which occurs when fuel is burned, results in the emission of flue gases. The composition of these gases includes CO₂, as well as CO, SO_x and NO_x. The Test 1 gas analyser (LLC “Bonare”, Novosibirsk, Russia) to analyse the composition of flue gases was used. This device is equipped with electrochemical sensors that can detect ultra-low concentrations of NO_x and SO₂ gases. For CO and CO₂ gases, which are present in higher concentrations, optical sensors were used.

The phenomenon of the greenhouse effect is associated with the emission of CO₂; meanwhile, sulphur and nitrogen oxides have been demonstrated to exert deleterious effects on human health. The analysis of the concentrations of components in flue gases was conducted on an experimental stand, which incorporated a tubular muffle furnace and a gas analyser for the determination of ultra-small gas concentrations. The bench was equipped with a probe for gaseous sampling and connected to a computer and a coordinate mechanism for automatic introduction of a 0.3 g portion of fuel inside the muffle furnace. During ignition and combustion, the flue gases were channeled through the probe and into the gas analyzer, which included dehumidification and filtration steps, to measure their concentrations. After each experiment, the gas ducts and muffle furnace chamber were flushed with fresh air to eliminate any remaining sample residue. The most important indicator was the repeatability of the experimental results, which was ensured by conducting each experiment at least twice, and the inaccuracy was no more than 3%. A comprehensive description of the experimental setup, accompanied by a schematic representation, can be found in our publication [36,37].

3. Results and Discussion

3.1. Combustion of Individual Fuels

As illustrated in Figure 3, the way fuels are combusted is characterised by the profiles of Weight, Deriv. Weight, and Heat Flow curves. The thermogravimetric analysis of fuel combustion entails subjecting the sample weight to a heating rate of 20 °C/min. The entire heating process can be subdivided into three distinct temperature intervals: the first one corresponds to moisture removal; the second one corresponds to thermal decomposition accompanied by intensive release of volatile substances and their combustion; the third stage corresponds to the ignition and combustion of coke residue. The whole process in the temperature range of 25–800 °C is accompanied by fuel mass loss, changes in the rate of fuel mass reduction, and thermal effects.

At the first stage, in the temperature range of 25–110 °C, moisture removal from coal and sewage sludge takes place with a slight decrease in mass (3.76% from coal and 2.52% from sewage sludge) (Figure 3a). At further heating, thermal decomposition of fuels occurs in the temperature range of 110–365 °C for coal and 110–260 °C for sewage sludge. This process is accompanied by mass reduction by 17.7% for C and 5% for SS. In Figure 3, this temperature interval is expressed by an increase in the mass loss rate and heat flux intensity. The observed exothermic effect is due to partial combustion of volatile substances (Figure 3c).

The basic combustion process of the fuel begins with the ignition of coal coke and sludge coke, which initiates the release and combustion of volatile substances, as well as an increase in temperature. The residual volatile substances released at higher temperatures are burnt out together with the coal coke and sludge coke. The process under investigation takes place within the temperature range of 365–560 °C for coal and 260–675 °C for coke sludge. The mass loss during the combustion of coke residue in coal was 73.3 per cent, while in sewage sludge it was 39.7 per cent (Figure 3a). Smaller mass loss at burning of SS sample is connected with its high ash content (Table 1).

During the combustion of the coal coke, the maximum rate of mass loss was recorded as 25.2% per minute at a temperature of 430 °C, while the maximum rate of heat release was determined to be 6.1 W/g at 433 °C. The coal combustion process was completed at 560 °C. The coal combustion index was determined to be 1.4 min⁻² °C⁻³.

The sludge coke combustion was accompanied by a peak mass loss rate of 4.9 %/min at 330 °C. A peak heat flux rate of 1.2 W/g was achieved at 338 °C. Complete SS burnout occurred at 675 °C, with a combustion index of 0.3 min⁻² °C⁻³.

A thorough examination of the collected data reveals that the combustion of sewage sludge, in contrast to coal, exhibits a prolonged combustion process. A notable benefit of sewage sludge over coal is its low ignition temperature. In addition, a multitude of other parameters associated with SS combustion are found to be considerably lower than those associated with coal combustion (see

Table 3. Combustion parameters of C, SS and their mixtures.

Characteristics	Biofuels
	C	75% C + 25% SS	50% C + 50% SS	25% C + 75% SS	SS
Ti, °C	365	355	291	274	260
Δmmax1, %/min	-	-	3.9	4.1	4.9
TΔm1, °C	-	-	340	333	330
HFmax1, W/g	-	-	1.2	1.2	1.2
THF1, °C	-	-	347	341	338
Δmmax2, %/min	25.2	19.1	11.7	7.4	1.4
TΔm2, °C	430	418	422	424	535
HFmax2, W/g	6.1	3.9	2.4	1.7	-
THF2, °C	433	427	428	430	-
Tb	560	571	586	594	675
S, min−2 °C−3	1.4	1.0	0.7	0.5	0.3

). This finding serves to underscore the efficacy of SS combustion in a mixture with a more highly reactive fuel, such as coal.

3.2. Combustion of Coal and Sewage Sludge Based Mixtures

As shown in Figure 4, the combustion behavior of the mixtures is depicted through the Weight, Deriv. Weight, and Heat Flow curve profiles. The combustion of the mixtures occurred under conditions of slow heating in an airflow. The heating rate was 20 °C/min. During the heating process, three main temperature intervals were observed, corresponding to the removal of moisture, the thermal decomposition of the mixture, and the combustion of coke residue and the residual volatile substances. The primary parameters associated with mixture combustion are delineated in Table 3.

When the mixtures are heated, combustion is expressed by two extrema of the Deriv Weight and Heat Flow curves, (Figure 4b,c) in contrast to the separate combustion of the components (Figure 3b,c). This effect is explained by the fact that the components in the mixtures burn in their own temperature regions, so they have two extrema each.

In the mixture composed of 75% coal and 25% sewage sludge in the low temperature region, SS combustion is a less pronounced character and does not appear as an extremum, thus the peak values of Δm_max1 and HF_max1 could not be recorded. The carbon residue ignition begins at 355 °C. The maximum rate of mass loss occurred solely in the higher temperature range, reaching 19.1 %/min at 418 °C, which corresponds to the combustion of the coal coke residue and the subsequent burning of volatiles. Concurrently, the peak value of heat flux intensity was recorded at 3.9 W/g at 427 °C. The combustion process ended at a temperature of 571 °C. The combustion index decreased slightly compared to coal and was 1 min⁻² °C⁻³.

When the proportion of sewage sludge in the mixture is increased to 50% in the temperature region of 250–350 °C, SS combustion is observed. This is expressed by a peak mass loss rate of 3.9 %/min at 340 °C and a heat flux intensity of 1.2 W/g at 347 °C. The ignition of the coke residue in the mixture occurs at 291 °C. The second peak in Figure 4 represents the combustion of the coke residue and the remaining volatiles, with a maximum mass loss rate of 11.7 %/min occurring at 422 °C and a heat flux peak of 2.4 W/g at 428 °C. Importantly, the completion temperature of the combustion process is higher than that of coal, reaching 586 °C. As a result, the combustion index dropped to 0.7 min⁻² °C⁻³.

When the proportion of sewage sludge in the mixture is increased to 75%, the ignition temperature drops significantly to 274 °C. During the combustion of the coke residue and sewage sludge volatiles, the peak mass loss rate was recorded at 4.1 %/min at 333 °C, along with heat release peaking at a heat flux intensity of 1.2 W/g at 341 °C. The combustion of coke residue and coal volatiles takes place at higher temperatures, indicated by a distinct peak in the mass loss rate of 7.5 %/min at 424 °C, accompanied by a heat flux intensity of 1.7 W/g at 430 °C. The overall combustion process of this mixture concludes at a higher temperature compared to other mixtures and pure coal, reaching 594 °C. As a result, the combustion index for this mixture decreased to 0.5 min⁻² °C⁻³.

It has been established that the addition of coal to sewage sludge enhances the combustion process and the burnout of waste. Concurrently, the temperature at which coke residue ignition occurs is reduced. The most energy-efficient proportion of SS to coal that can be added is 25%, as it has a positive effect on the combustion process of the mixture. Qin S. et al. [38] investigated the joint combustion of SS and high ash coal in their work. It was found that the addition of SS to coal had a positive effect on reducing the ignition temperature of the mixture, which confirms our results.

The low value of temperature at which SS ignition takes place and the high content of volatile substances have a positive effect when adding 25% SS to 75% coal on the earlier ignition of the mixture, which leads to a more complete burnout, which in turn increases the energy efficiency of this method of sewage sludge utilisation.

3.3. Synergetic Effects in Combustion of Mixtures

As illustrated in Figure 5, a comparison of the profiles of the curves Deriv. Weight curves gained by the experiment and calculated for all three mixtures.

The main combustion parameters of the mixture can be determined by knowing the relevant parameters of each component if the mixture exhibits additive combustion, as in the case of a mixture containing 75% coal and 25% sewage sludge (combustion profiles are shown in Figure 5a). There is no significant separation between the DerWeig_exp and DerWeig_est curve profiles, indicating additive combustion.

For the mixture composed of 50% coal and 50% sewage sludge, synergetic combustion is exhibited in the low-temperature region during the process of burning coke residue and a portion of the volatile components from the sewage sludge (see Figure 5b). The calculated values of the peak mass loss rate were 4.6 %/min at 342 °C, and the experimental values were 3.9 %/min at 340 °C. Concurrently, the experimental value of Der.Weig._max1 decreased by 18%, signifying a decline in the combustion rate. In the temperature region characteristic for the combustion of coke residue and volatile substances of coal in the mixture, negative interactions were also observed, which affected the decrease of Der.Weig._max2 from 13.3 to 11.7 %/min (Figure 5b).

When burning a mixture based on 25% coal and 75% sewage sludge, interactions between the components are also observed negatively affecting the combustion rate (Figure 5c). Furthermore, at the combustion of coke residue and volatile substances from sewage sludge in the lower temperature region, a decrease of Der.Weig._max1 from 4.5 %/min (obtained by calculation) to 4.1 %/min (obtained during the experiment) is observed. In the temperature range which corresponds to the combustion of coke residue and part of coal volatiles, negative interactions are also observed, which influenced the decrease of Der.Weig._max2 from 7.9 %/min (obtained by calculation) to 7.4 %/min (Figure 5c).

The findings of this study demonstrate that the interactions of mixture components exert a detrimental influence on the combustion process, as evidenced by a decline in the maximum combustion rate. The impact of high ash content in sewage sludge, comprising mineral particles, is a notable factor in this regard. These mineral particles have been observed to impede the penetration of oxidants into the carbon portion of coal, thereby hindering the combustion process. Additionally, the presence of high levels of volatile substances has been shown to be ineffective in enhancing the combustion rate of the mixture. Other authors obtained similar results. Fu et al. [25] established in their study that the synergistic effect in the combustion of coal sludge and SS mixtures was observed in the low-temperature range. This finding suggests that mixtures may possess enhanced flammability. Furthermore, they concluded that this method of waste disposal is a promising avenue for energy extraction and environmental benefits.

It is more likely that at high content of high-ash component sewage sludge the heat released during combustion of the coal component was partially spent on heating of inert mineral components of sewage sludge. This reduced the actual local temperature in the combustion zone and possibly the corresponding combustion performance.

3.4. Analysis of Concentrations of Gaseous Substances

As illustrated in Figure 6, the experimental findings concerning the concentrations of carbon monoxide, carbon dioxide, nitrogen, and sulphur oxides, which are produced during the combustion of various fuels and fuel mixtures, are presented.

As demonstrated in Figure 6a, at coal combustion the concentration of CO₂ in flue gases is 11% lower than at combustion sewage sludge. Furthermore, a rise in the proportion of SS within the mixture has been shown to result in an increase in the concentration of carbon dioxide (Figure 6a).

The formation of carbon monoxide during the process of coal combustion is lower than during the combustion of sewage sludge (by a factor of 4). This phenomenon may be attributed to incomplete combustion of carbon-containing fuels resulting from a deficiency of oxidant within the combustion zone. The latter, in its turn, can be a consequence of the low-porous or narrow-porous structure of the combustible substance with high content of mineral components. In addition, mineral substances can inhibit the combustion process. When burning C, the carbonaceous mass is known to form a highly porous structure, easily accessible to oxygen, which provides a high concentration of oxygen in the combustion zone and, accordingly, the completeness of combustion of coal particles. A. Sever Akdağ et al. [39] in their work investigating the combustion of coal and sewage sludge found that an increase in the mass fraction of SS in the mixture with coal affects the increase in CO concentrations.

The concentration of nitrogen oxides in flue gases at coal combustion was found to be 2.6 times higher than at SS combustion (Figure 6b), despite their content in initial coal being 6 times less (Table 1). The combustion of fuel gives rise to three types of nitrogen oxides: fuel, high-temperature and fast nitrogen oxides. Of these, high-temperature nitrogen oxides have been found to have the greatest impact on the increase in NO_x concentration in flue gases (Figure 6). Therefore, it is customary to reduce the flame temperature in the furnace of a coal-fired boiler by transferring to staged or low-temperature combustion of fuel. As the SS share in the mixture increases, the concentration of nitrogen oxides decreases (Figure 6b).

The concentration of SO₂ in flue gases from coal combustion was found to be 15 times higher than that from SS combustion, despite the initial coal containing five times less sulphur than SS. Increasing the proportion of SS in the mixture has been shown to result in a decrease in the concentration of sulphur oxides in flue gases. The reduced concentration of sulfur oxides in flue gases during SS combustion appears to be associated with the ability of mineral substances (primarily calcium compounds) to chemically bind them in the form of metal sulfates.

4. Conclusions

The study of joint combustion of sewage sludge and coal, with consideration for the concentrations of gaseous substances in flue gases, enabled the following conclusions to be drawn:

• Sewage sludge has been found to contain high ash content, a high content of volatile substances, and a low heat of combustion, in contrast to coal.
• The chemical composition of sewage sludge was found to be dominated by silicon (in the form of quartz and silicates), calcium (in the form of calcite, gypsum, and apatite), and iron (in the form of oxides and silicates). The inorganic residue was found to be primarily silicon oxide, with a content of 45.1%, followed by phosphorus oxide (17.7%), calcium oxide (11.6%), and iron oxide (11.0%). Nevertheless, SS ash after combustion should be disposed of in special landfills where ash after combustion of thermal coals is disposed of, in order to avoid negative consequences when heavy metals are released into groundwater.
• The ignition temperature of the sewage sludge is 40% lower than that of coal, and its combustion index is 4.6 times lower.
• It has been established that for the combustion of sewage sludge with such calorific properties, the most suitable fuel mixture is based on 75% coal and 25% sewage sludge. This mixture has an additive burning character, while compared to coal, it has a lower ignition temperature. The remaining parameters remain relatively stable when compared to other mixtures that contain greater quantities of sewage sludge.
• The addition of 25% sewage sludge to coal has been shown to affect the reduction of nitrogen oxides and sulphur oxides.
• Adding 25% of sewage sludge to coal allows reducing coal consumption without significant reduction of combustion parameters of the mixture, which indicates an increase in energy efficiency of SS utilisation by combustion with coal.